As known, high-frequency electronic devices play an important role in modern technologies. With substantial increase of operational frequencies, a variety of problems are possibly caused in printed circuit boards (PCBs) themselves, configuration of integrated circuits (ICs) in PCBs, and interconnection structures such as back planes or connectors for physically connecting to active devices or power supplies. Due to the electric properties of the signal transmission structures, problems in power integrity and electromagnetic interference, etc., might occur or get worse. Therefore, manufacturing difficulty and hardware cost would rise.
For solving the above-mentioned problems, factors associated with high-frequency signal transmission are studied and controlled. For example, characteristic impedance of a transmission line is an important controllable factor for improving signal transmission efficiency.
So far, a variety of measuring techniques adapted for high frequency signals have been proposed to control characteristic impedance of a transmission line. For example, in one of the measuring techniques, a Sub Miniature A (SMA) connector is disposed at an edge of a PCB, and used as an input and output point of a measurement signal. Unfortunately, the design of the connector is confined due to the size requirement. As known to those skilled in the art, for SMA I/O connectors with 3.5 mm or 2.92 mm female connectors, there should be a clearance of 12 mm or more between terminals of two connectors to avoid unexpected contact. In another example, for Bayonet Neill-Concelman (BNC) coaxial cable connectors, a terminal of a connector needs to be kept 20 mm or more away from a terminal of another connector to make connection feasible.
To avoid such confining conditions, a high frequency and low cost probe means is proposed to measure properties of high-speed PCB signals lines. Please refer to FIG. 1. A probe 10 includes an SMA connector 12 and a needle 14. When a Vector Network Analyzer (VNA) is used to execute measurement of characteristic impedance, e.g. S-parameter, two probes 10 are disposed at two ends of a high-speed/high-frequency signal line, respectively, wherein one of the two probes 10 serves as an input point and the other serves as an output point. Accordingly, it is understood that the measured characteristic impedance would contain mixed characteristic impedances of internal elements, connecting coaxial cables, connectors, probes and high-speed PCB signal lines.
Since it is necessary to specifically realize the characteristic impedance of a specific target to be measured, e.g. a high-speed PCB signal line, calibration of the instrument is essential to measurement precision. When conducting calibration, a Vector Network Analyzer measures an object whose characteristic impedance is well known or partially known first. The measured data are then referred to for correcting deviation of the system so as to isolate the desired characteristic impedance of a high-speed PCB signal line from the mixed one containing the characteristic impedances of internal elements, connecting coaxial cables, connectors and probes. In other words, as illustrated in FIG. 1, a reference plane can be established at the needle tip 14A of the probe 10.
Conventionally, four measuring ways, including open-circuit, short-circuit, applying load and transmitting thru, are used for measurement of characteristic impedance for calibration in order to move the reference plane to the needle tip of the probe. Generally, the measuring means are provided by VNA manufacturers, and can only be used with some specific probes for calibration unless specifically prepared carriers are used. The carries need to be specifically designed for different circuit boards, so time and cost would be additionally spent. Furthermore, the carriers might not be adapted for measurement of other kinds of objects.
In other words, the conventional measuring means are limited in many ways and lack of flexibility.